Researchers Explains Coral Reef Bleaching

The Great Barrier Reef, which stretches for more than 1,400 miles off Australia’s northeastern coast, has been called the largest living structure on earth. But the journal Nature reported last month that the reef is rapidly becoming the world’s largest dying structure. This assessment was based on a survey led by biologist Terry Hughes, the director of the Arc Center of Excellence for Coral Reef Studies at James Cook University in Queensland Australia. Hughes and his colleagues found that two-thirds of the northern sector of the reef has been badly damaged by a massive bleaching event that occurred over a period of several months last year.

This week, Hughes and his team are delivering more grim news. An almost equally massive bleaching event is happening right now along a 500-mile portion of the reef. This ongoing event is occurring in the central sector of the reef, just south of last year’s destruction. Together, these back-to-back events comprise the largest coral bleaching ever recorded, impacting two-thirds of the Great Barrier Reef.

In an interview with Yale Environment 360, Hughes lays the blame for the recent destruction in the Great Barrier Reef squarely on warming waters caused by climate change. The catastrophic damage to Australia’s reefs is part of a global phenomenon that is threatening the survival of coral worldwide, Hughes says, and is a clear warning that we need to rein in greenhouse gas emissions: “We simply cannot afford to continue with business as usual. We are very concerned that these events are getting more frequent. The reef simply won’t come back if we have a bleaching event every other year.”

Yale Environment 360: Your findings detailing the extent of the damage to the Great Barrier Reef were shocking. How have Australians been reacting to the news that much of their nation’s most iconic natural feature is either dead or dying?

Terry Hughes: There is a feeling of dismay in the science community as we continue to quantify the huge geography of this recurring phenomenon. When I show the aerial clips of our survey during public lectures, the room goes silent. People are truly shocked and horrified by the extent of the damage. I’ve had people in the front rows of my lectures crying. The positive side of that is that we are seeing that people really care about the Great Barrier Reef, and they are searching for answers and also putting pressure on governments to do something about it.

e360: Could you describe your bleaching study?

Hughes: The paper talked about three events — in 1998, 2002, and 2016. The basic research questions that we are asking are: What is the geographic footprint of the bleaching? Where is it occurring? And where is it the most severe? To answer these, we conducted surveys from the air. I personally have spent eight days flying in a small plane and helicopter, covering in all about 5,000 miles in the air. We fly very low and as slowly as we can over individual reefs. Last year, we surveyed more than 1,000 of the Great Barrier Reef’s nearly 3,000 reefs. It is incredibly challenging and emotionally exhausting to fly for hours over, say, 200 reefs in the worst-affected area, where reef after reef is 60 to 80, or more than 80, percent bleached.

e360: When did you conduct the survey?

Hughes: The timing of the aerial surveys is critically important. If you go too early, the bleaching is still building, and if you go too late, the corals start to die. And once the white bleached corals die, they go dark brown, get covered in seaweed, and you can’t see very much from the air. So there is a window of only a couple of weeks when it is ideal for doing the aerial surveys.

e360: How do you determine how many of the corals that bleach actually end up dying?

Hughes: From the air, we categorize the corals as to whether or not they are bleached. At the same time, we put about 100 researchers underwater to test the accuracy of the aerial scores by measuring in much closer proximity the proportion of corals that were bleached, and how many are ether dead or dying.

So last year, for instance, 55 percent of the reefs that we surveyed were severely bleached — that was four times more than happened during either 2002 or 1998. We then went back six months later basically to see what was left. In the northern 500 miles, two-thirds of the coral that were alive in March/April were dead by October/November — so a huge loss.

e360: You just announced a fourth mass bleaching, one that happened in the past few months.

Hughes: We now have a 2017 event, which is not quite as bad as 2016, but certainly worse than the first two events that we studied [in 1998 and 2002]. That is significant because it postpones any hope of recovery. The current bleaching occupies a different geographical footprint from last year, which is bad news because it means between last year and this year a much greater extent of the Great Barrier Reef has now been damaged. In 2017, the hot water was in the middle of the Great Barrier Reef, the central section; last year it was in the north.

If you look at the cumulative loss from these two events, the northern two-thirds of the Great Barrier Reef have now been severely damaged. The bottom third escaped bleaching both years because the waters were cooler there. But we are very concerned that these events are getting more frequent.

e360: Did the kinds of mass bleaching that we are seeing today also happen in the past?

Hughes: The phenomenon that we are seeing now is a very modern issue for reefs. Until an El Niño event in 1982-83, mass bleaching at the scale of, say, the Caribbean basin or the Western Pacific or the Indian Ocean was unheard of. The first completely global bleaching event was in 1998. That was a wake-up call for people here in Australia, because that was the first time ever that the Great Barrier Reef had bleached.

e360: What actually causes coral bleaching?

Hughes: The three main drivers of degradation are pollution, overfishing, and, increasingly, climate change. I did my PhD at Johns Hopkins University in Baltimore looking at Jamaican coral reefs. Caribbean reefs became degraded much earlier than elsewhere due to runoff following land clearing, mostly for sugar cane, and they are also very heavily fished. So there are few fish left, which means that when the coral die, they are rapidly covered with seaweed rather than with new corals — because there are not enough herbivores, like fish or sea urchins, to eat the seaweed. Tragically, the Great Barrier Reef is rapidly catching up with the Caribbean. But the cause in Australia is different — in our case it is entirely due to the warming water.

‘We are on a trajectory toward more, and more frequent, mass bleaching events.’

e360: What impact has El Niño played in the recent mass bleaching?

Hughes: El Niños cause abnormally warm conditions, La Niña does the opposite. The 1982-83 and 1998 event were both associated with strong El Niños. The 2016 El Niño event caused global-scale bleaching in three quarters of the world’s coral reefs spread over the Indian Ocean and the Pacific. Prior to 1982-83, El Niños obviously occurred on a regular cycle every five years or so. But they did not cause bleaching. El Niños have only become dangerous to coral in recent decades because of the rising underlying water temperature due to global warming.

And now we are seeing massive bleaching events outside of El Niño conditions. The reality is that La Niña years now are warmer than El Niño years were say 25, 30 years ago, due to global warming. The second event in 2002, and the fourth event in 2017, were not during El Niño conditions; the water, even in intermediate years, is just too damn hot. So we are on a trajectory toward more, and more frequent, mass bleaching events. And the gap between these recurring events is getting shorter and shorter — tragically we have had no gap at all between 2016 and 2017. So the same corals that are still suffering from the aftermath of last year’s bleaching are now re-bleaching, and many of them are dying because they are very stressed. So I would argue that we are running out of time.

e360: Running out of time?

Hughes: There is a narrow window of opportunity that is closing to deal with greenhouse gas emissions. And the obvious mechanism for doing that is the COP21 [United Nations] process, which has set a target of 1.5-degree or 2- degree C [increase] that many countries have signed up for. Australia signed up for the 1.5-degree target, but unfortunately we are not doing very much to achieve that target. Really what Australia should be doing, in my opinion, as the country responsible for the stewardship of the Great Barrier Reef is taking a leadership role in transitioning away from fossil fuels, but sadly that hasn’t occurred.

e360: Is what is happening to Australia’s reefs worse than elsewhere?

Hughes: The Great Barrier Reef is actually quite typical. Globally, most coral reefs have now bleached three or four times since the 1980s. A few have bleached severely up to nine times. And a very few have so far escaped bleaching, but that number is getting smaller and smaller.

e360: How do bleaching events unfold? Does the entire reef bleach at once?

Hughes: It is a bit of a myth that people think of bleaching as being a sharp event in time. The reality is that it unfolds slowly over several months or longer and it goes through various phases. Different corals have different degrees of susceptibility to bleaching, and they also have different abilities to regain their color and survive if they do bleach.

So let me try to describe the process as it unfolds on the Great Barrier Reef. In February, as the temperatures are rising toward the mid-March peak, heat stress is building and the susceptible species start losing color. Maybe two weeks later they are white and the tougher species that are more thermally tolerant, they start to pale and if the heat is extreme enough even they will turn white. Last year the bleaching was so severe that we saw very high rates of bleaching and mortality in coral species that are very tough and normally don’t bleach.

e360: Which species are the most vulnerable to bleaching?

Hughes: The worst affected were the branching corals in the genus Acropora, which is the most diverse genus and is typically comprised of three-dimensional corals that grow quickly. They dominate Indo-Pacific coral reefs. The other group that we saw with very high mortality is the so-called soft-corals — and many of them just melted away in the extreme temperatures.

The corals that survived are a different set of species. Many of them are mound-shaped like brain corals and a genus called Porites — the tougher, slower-growing species. But even some of those died. We saw 50- and 100-year-old corals dying because of last year’s bleaching event. From this, we have concluded that the mix of species in the northern Great Barrier Reef has changed forever. These slow-growing corals will be replaced by the faster-growing weedy ones. But even those faster-growing species need a decade to recover, and obviously we didn’t get a decade between 2016 and 2017, so there is no time for recovering. The reef simply won’t come back if we have a bleaching event every other year.

Between 2016 and 2017, nearly two-thirds of the Great Barrier Reef has undergone bleaching due to warm ocean temperatures.

e360: What do we know about the capacity of reefs to naturally recover from bleaching?

Hughes: Most of what we know about recovery on reefs comes after cyclones or hurricanes. A hurricane typically has a footprint of about 100 kilometers in width. They form offshore and they come to the mainland, crossing the Great Barrier Reef. The recovery of reefs that have been damaged by that cyclone can happen from [coral] larvae arriving from the north or from the south on either side of that storm track. So the scale of dispersal that is needed is about 50 kilometers coming in from either side.

The damage we’ve seen in the north and now in the center of the Great Barrier Reef means that the brood stock in that vast area, which is 1500 kilometers in length, is very depleted. It begs the question of where the larvae are going to come from. We know quite a bit about recovery on the scale of a cyclone track. But the analogy I make here is that it’s like ten category 5 cyclones have come ashore simultaneously holding hands between New Guinea and where I live in Townsville [in northern Queensland]. So in terms of the dynamic of that recovery — we are really in uncharted territory as to how long that would take.

e360: Some have suggested that coral from deeper reefs is less affected by bleaching and can help repopulate the shallower reefs. Is this realistic? What has the impact of the last two events been on the deeper-dwelling coral?

Hughes: The deepest we investigated last year was 40 meters [131 feet] underwater. We found bleaching all the way down, but it diminished with depth. Nonetheless, at some of our 40-meter sites, we saw that as many as 30 percent of the corals were bleached.

The capacity of the deep reefs to act as a source of larvae and recolonize the shallow reef is quite limited. There is a different assemblage, a different mix of species. The species that are common in shallow water are rarely common in deep water, and obviously their physiologies are different, they are accustomed to very different light levels. So if you transport a coral up from the deep into the shallow, they often die because their physiology is adapted to the deep. To do so may help a bit, but only for species with very broad depth ranges. It is not going to make a huge contribution to the recovery.

‘There is a rapidly closing window where we still have the opportunity to address this issue.’

e360: What about artificial breeding? Some researchers, like coral biologist Ruth Gates in Hawaii, are working on breeding coral in the lab that are resilient to warmer water and then introducing them into the wild.

Hughes: The idea is you raise coral in an aquarium at elevated temperatures so they are adapted to those temperatures, and then you put them out into the wild and over time they infiltrate into the wild population and replace the more susceptible individuals in that population. I have a problem with the scale of that because the size of the wild populations is vast, and adding, say, 1,000 aquarium-bred individuals into the wild is not going to make a significant difference to the gene pool.

e360: Is there anything that we humans can do at this stage to protect reefs in Australia and beyond?

Hughes: The situation is dire. But I don’t want to convey a message of despair or suggest that it is too late. What I want to say is, “Look this is really very serious, but we shouldn’t give up on the Great Barrier Reef, or on coral reefs globally.” There is a rapidly closing window where we still have the opportunity to address this issue. Cutting greenhouse gas emissions is the way. We simply cannot afford to continue with business as usual.

Aerial photographs of current coral bleaching along a 500-mile stretch of the Great Barrier Reef. The bleached sections appear white or yellow compared to the healthy coral surrounding it.

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